Vapor compression systems are often used in heat pumps to, for example, heat and cool air, water, or other fluids. Most simple compression systems operate at a subcritical state where the refrigerant in the vapor compression system is maintained at a combined liquid-vapor state. To provide an additional degree of freedom over compression system control, however, a user may choose to use a transcritical compression system, which allows the refrigerant to reach a super-critical vapor state.
If a transcritical vapor compression system is used as a heat pump in a heat pump water heater, the water heater should undergo a warm-up procedure at startup to bring the heat pump to a steady state at which the components in the heat pump are at their target states. Variable overshoots may occurs in the heater during the warm-up procedure, causing the heater to shut down in an attempt to protect the heater. Further, signals from the expansion valve and the water pump may be sequenced in a manner that undesirably reduces the operating efficiency of the heater. Heat pumps incorporating transcritical vapor compression systems may be particularly vulnerable to shutdowns caused by improper startup due to their extra degree of freedom.
There is a desire for a method that brings the heat pump in the water heater to a steady state without causing variable overshoots or improper system sequencing that reduce energy efficiency.